1. Field of the Invention
The present invention relates to a method of preparing a solid composition comprised of a thermoplastic molding compound based on polyphenylene ethers (PPEs), on the one hand, and a sulfur-vulcanizable rubber component, on the other; and the product obtained from the method.
2. Discussion of the Background
Often a single material cannot provide all the properties which are desired of an article of manufacture. Thus, a single chemical substance will not provide an article which simultaneously has high strength and rubber elasticity, or high hardness and stiffness, on the one hand, and high traction on the other.
In order to provide structural component parts with properties which a single constituent cannot alone contribute to the component, different parts are manufactured from combinations of pieces which are comprised of several different materials. Often a necessary precondition for good performance characteristics of such articles is good adhesion between the pieces comprised of different materials.
Articles which are formed of combinations of thermoplastics and rubber are ordinarily bound to form an integral structure by mechanical clamping or the like, adhesive bonding, or covulcanization using special covulcanization auxiliary agents.
The force of bonding achieved by mechanical clamping or the like is adequate to withstand only low stresses. On the other hand, it is very costly to use the adhesive bonding technique in the mass production of articles. Moreover, the use of adhesives often leads to additional problems with materials.
The best bonding technique is to covulcanize the stiff molding compound and the rubber material. The state of the art with this technique is that either the surface of the stiff molding compound must be pretreated and/or the rubber mass must be specially treated.
One proposed treatment of the surface of the thermoplastic component (L. H. Nitzsche, 1983, Kautschuk+Gummi, Kunststoffe, 36:572-576) consists of painting the thermoplastic with an aqueous solution of vinylpyridine latex, resorcinol, and formaldehyde (see "Keilriemen", a monograph from Arntz-Optibelt-Gruppe Hoexter, pub. Verlag Ernst Heyer, Essen, FRG, 1972, p. 83).
Another proposed treatment involves coating the thermoplastic surface with a gasoline solution of isocyanates (Bayer-Taschenbuch fuer die Gummi-Industrie, 1963, p. 254) One of the methods involves the treatment of the rubber mass with adhesion promoting additives. Suitable such additives are, e.g., combinations of resorcinol, formaldehyde-dispensing agents, silicic acid, zinc oxide, and fatty acids (W. Kleemann, Mischungen fuer die Elastverarbeitung, Leipzig, 1982, p. 300).
It is also known that SBR and EPR rubbers, as well as polybutadienes, show an unexpected, excellent adhesion to certain plastics upon relatively long thermal treatment. These certain plastics, distinguished by having recurring units of the formula: ##STR3## comprise poly(2,6-dimethyl-1,4-phenylene ethers), polysulfones, and polycarbonates. For comparative purposes it should be noted that polystyrenes, which do not contain this structural unit, have an adhesive force which is smaller by a factor of &gt;6 (P. Dreyfuss and M. L. Runge, J. Appl. Polym. Sci., 23:1863-1866). These authors assert that this method achieves outstanding binding of the elastomeric and plastic layers in many cases, even if the elastomer has no double bonds (as is the case with EPR rubber). However, the method of thermal adhesion on the part of certain plastics has the following major disadvantages:
The adhesive strengths achieved still do not appear to be adequate to the extent that such binding systems can be used industrially; PA1 Pure polyphenylene ethers play an economically minor role, because of their poor processibility and because of their inadequate impact strength. All PPE-containing polymer mixtures contain added polymers which have substantially inferior binding properties. The above-described method is not considered suitable for economically important polymer mixtures, all of which have styrene polymers present in the amount of &gt;10 wt. %; PA1 For industrial applications, the rubber employed contains a filler or fillers. One would expect, that the filler additive, however, adversely affects the adhesion properties of the rubber in comparison to filler-free rubbers; PA1 The resulting compound is not stabile to the action of hydrocarbons; and PA1 The treatment times are extremely long. Multi-hour processing operation times cannot be integrated with modern engineering arrangements for manufacturing molded parts. PA1 (1) The method should be applicable not only to PPEs, but also in general to polystyrene-containing and to hydrocarbon-containing PPE molding compounds. PA1 (2) The adhesion should be capable of being developed within a few minutes. PA1 (3) Adhesive strengths should be achieved which are better than those with known bonding systems. PA1 (4) The bonding system should be resistant to hydrocarbons. PA1 (a) 100 parts by weight of polymers of ortho-substituted phenols having the formula: ##STR5## where R.sub.1 and R.sub.2 independently represent a methyl group or hydrogen, and R.sub.3 and R.sub.4 represent sn alkyl group with up to 6 carbon atoms, or R.sub.3 represents hydrogen and R.sub.4 represents a tertiary alkyl group with up to 6 carbon atoms; PA1 (b) 0-20 parts by weight polyalkenylenes; and PA1 (c) 0-100 parts by weight styrene polymers;
Under these conditions it is unsurprising that there has been no further refinement of this method nor any industrial adoption of it.
When one examines the method, despite these objections, one concludes that the authors aimed at a method for producing a physical bond between thermoplastics and synthetic rubbers. In particular, an objective of the method is to prevent cold flow on the one hand and to avoid chemical bonding on the other hand, for It has been Found, that as the degree of crosslinking of the rubber increases, the adhesive force decreases (A. Ahagon and A. N. Gent, 1975, J. Polym. Sci., Polym. Phys. Ed., 13:1285 Abstract).
It has been learned by conducting tests that the adhesion is not at all as good as represented (see the Comparison Tests, infra). In certain individual cases the separation forces are immeasurably small, because the rubber becomes crumbly during the thermal treatment. A need therefore continues to exist for an improved method of bonding PPE containing thermoplastic molding compounds to elastomeric rubbers.